1. Field of the Invention
The present invention relates to a power supply. More specifically, the present invention discloses a method of power factor correction without using current sensing or a multiplier by using a generated predictive pulse to charge and discharge the power factor correction (PFC) inductor.
2. Description of the Prior Art
A conventional power factor correction PFC technique is to have the power supply input resemble a resistor. This is achieved by programming the input current in response to the input voltage. With an in phase relationship between the voltage and the current the input will be resistive. This provides a power factor equal to one.
However, distortion or phase displacement in the input can occur if the ratio is not constant. Distortion or phase displacement will negatively affect the power quality.
The conventional method of power factor correction increases the power factor to ensure that the phase angle between the current and the voltage approaches zero. Unfortunately, this technique is not always achievable in practice.
As shown in FIG. 1A, in an alternating current circuit, the current signal 120 will follow the voltage signal 110 if the load is simply resistive. This means that when the maximum voltage is across the load the maximum current flows. When the voltage reverses so, does the current.
However, as shown in FIG. 1B, complex alternating current loads are not always this simplistic. In actuality the current signal 120 does not precisely follow the voltage signal 110 because the load is inductive, capacitive, or a combination which varies over time. As the complexity of the load increases, the phase displacement between the current and the voltage increases. As a result, the power factor is reduced and the power supply is less effective.
Most conventional methods of PFC use a reference to generate a current that is power factor corrected. This reference is usually obtained by multiplying a scaled replica of the rectified line voltage vg times the output of the voltage error amplifier, which sets the current reference amplitude. In this way, the reference signal is naturally synchronized and proportional to the line voltage, which is the condition needed in order to obtain unity power factor.
One conventional PFC method is peak current control. In this method the switch is turned on via a constant clock signal and is turned off when the sum of the positive ramp of the inductor current and an external ramp or compensating ramp reaches the sinusoidal current reference.
Peak current control methods typically operate in continuous current mode CCM for reduced input filter requirement. The diodes used in the diode bridge can also be slow switching diodes because the bridge diodes only need to block at line frequency. However, the freewheel diode in the boost converter needs to be fast switching.
Refer to FIG. 1C, which is a diagram illustrating a current waveform of a conventional average current control method.
Another conventional PFC method is average current control. This method provides a better current waveform as the control is based on the average rather than the peak. The inductor current is sensed and filtered by a current error amplifier whose output drives a pulse width modulation PWM modulator. The inner current loop tends to minimize the error between the average input current and the reference. However, average current control operates on a constant switching frequency and has the same requirements for diodes as the peak current control method.
Refer to FIG. 1D, which is a diagram illustrating a current waveform of a conventional discontinuous current PWM control method.
A third type of conventional PFC is discontinuous current PWM control. In this method the switch is operated at constant on-time and frequency without an inner current loop. With the converter working in discontinuous conduction mode DCM, this control technique allows unity power factor when used with converter topologies like flyback. However, because of the discontinuous current, this method can cause harmonic distortion in the line current.
Therefore, there is need for a reliable and efficient method of power factor correction that is utilized in a power supply or power source system.